View source for Dactyl (moon)

{{Short description|Moon of asteroid 243 Ida}}

{{Infobox planet
| background = 
| minorplanet = yes
| mpc_name = (243) Ida I 
| alt_names = 1993 (243) 1
| name = Dactyl
| pronounced = {{IPAc-en|ˈ|d|æ|k|t|ᵻ|l}} {{respell|DAK|til}}<ref>{{OED|dactyl}}</ref>
| adjectives = Dactylian {{IPAc-en|d|æ|k|ˈ|t|ɪ|l|i|ə|n}}<ref>Edward Coleridge (1990) ''"The Argonautica" of Apollonius Rhodius'', p. 42</ref>
| named_after = [[Dactyls (mythology)|Dactyls]]
| image = File:Dactyl1.jpg
| image_scale = yes
| caption = Highest-resolution image of Dactyl, recorded while ''Galileo'' was about 3,900&nbsp;km away from the moon
| discovery_ref = 
| discoverer = Ann Harch
| discovery_site = ''Galileo'' spacecraft
| discovered = 17 February 1994
| mp_category = 
| orbit_ref = 
| epoch = 
| semimajor = 90 km at time of discovery
| perihelion = 
| aphelion = 
| eccentricity = 
| period = prograde, ca. 20 h
| inclination = ca. 8°<ref name="PetitDurdaGreenbergHurford1997p177" />
| satellite_of = Ida
| dimensions = 1.6×1.4×1.2 km
| mean_radius = 
| mass = 
| density = 
| surface_grav = 
| rotation = synchronous
| spectral_type = 
| abs_magnitude = 
| albedo = 
| single_temperature = {{convert|200|K|C F|abbr=on}}
| escape_velocity = 0.895m/s
}}

'''Dactyl''' ({{IPAc-en|ˈ|d|æ|k|t|ɪ|l}} {{respell|DAK|til}}), formal designation '''(243) Ida I''', is a small [[asteroid moon]] {{cvt|1200|by|1400|by|1600|m}}, in size) that orbits [[243 Ida]], a [[main-belt asteroid]]. It was imaged by the ''[[Galileo spacecraft|Galileo]]'' spacecraft on August 28, 1993; Dactyl was discovered while examining the delayed image downloads from Galileo on February 17, 1994. It was provisionally designated '''S/1993&nbsp;(243)&nbsp;1'''.<ref>{{Cite news |last=Clarke |first=Aubrey |date=August 28, 2021 |title=NASA Galileo Spacecraft Continues to Fulfill Many Tasks After Passing by Asteroid Ida 28 Years Ago |work=The Space Times |url=https://www.sciencetimes.com/articles/33119/20210828/nasa-galileo-spacecraft-continues-to-fulfill-many-tasks-after-passing-by-asteroid-ida-28-years-ago.htm}}</ref> The satellite was named after the [[mythology|mythical]] creatures called [[dactyl (mythology)|dactyls]], who, according to [[Greek mythology]], lived on [[Mount Ida]].<ref>{{Cite web |title=In Depth {{!}} 243 Ida |url=https://solarsystem.nasa.gov/asteroids-comets-and-meteors/asteroids/243-ida/in-depth |access-date=2022-08-11 |website=NASA Solar System Exploration}}</ref>

== Orbit ==
[[File:Dactyl potential orbits.svg|thumb|Diagram of potential orbits of Dactyl around Ida]]
Dactyl's orbit around Ida is not precisely known. ''Galileo'' was in the [[orbital plane|plane]] of Dactyl's orbit when most of the images were taken, which made determining its exact orbit difficult.<ref name="ByrnesD'Amario1994">{{harvnb|Byrnes|D'Amario|1994}}</ref> Dactyl orbits in the [[retrograde and prograde motion|prograde]] direction<ref name="PetitDurdaGreenbergHurford1997p179">{{harvnb|Petit|Durda|Greenberg|Hurford|1997|p=179}}</ref> and is inclined about 8° to Ida's equator.<ref name="PetitDurdaGreenbergHurford1997p177">{{harvnb|Petit|Durda|Greenberg|Hurford|1997|p=177}}</ref> Based on computer simulations, Dactyl's [[apsis|pericenter]] must be more than about {{convert|65|km|mi|abbr=on}} from Ida for it to remain in a stable orbit.<ref name="PetitDurdaGreenbergHurford1997p195">{{harvnb|Petit|Durda|Greenberg|Hurford|1997|p=195}}</ref> The range of orbits generated by the simulations was narrowed down by the necessity of having the orbits pass through points at which ''Galileo'' observed Dactyl to be at 16:52:05 UT on 28 August 1993, about {{convert|90|km|mi|abbr=on}} from Ida at longitude 85°.<ref name="PetitDurdaGreenbergHurford1997p188">{{harvnb|Petit|Durda|Greenberg|Hurford|1997|p=188}}</ref><ref name="PetitDurdaGreenbergHurford1997p193">{{harvnb|Petit|Durda|Greenberg|Hurford|1997|p=193}}</ref> On 26 April 1994, the [[Hubble Space Telescope]] observed Ida for eight hours and was unable to spot Dactyl. It would have been able to observe it if it were more than about {{convert|700|km|mi|abbr=on}} from Ida.<ref name="ByrnesD'Amario1994" />

If in a circular orbit at the distance at which it was seen, Dactyl's orbital period would be about 20 hours.<ref name="ChapmanKlaasenBeltonVeverka1994p455">{{harvnb|Chapman|Klaasen|Belton|Veverka|1994|p=455}}</ref> Its orbital speed is roughly {{convert|10|m/s|ft/s|abbr=on}}, "about the speed of a fast run or a slowly thrown baseball".<ref name="ByrnesD'Amario1994" />

== Origin ==
The origins of Dactyl are unclear, but two main hypotheses exist. The first is that Dactyl and Ida formed at the same time, and the second is that Dactyl was knocked loose by a later impact.<ref>{{Cite journal |last=Granahan |first=James |date=2002 |title=A compositional study of asteroid 243 Ida and Dactyl from Galileo NIMS and SSI observations |url=http://doi.wiley.com/10.1029/2001JE001759 |journal=Journal of Geophysical Research |language=en |volume=107 |issue=E10 |pages=5090 |doi=10.1029/2001JE001759 |bibcode=2002JGRE..107.5090G |issn=0148-0227|url-access=subscription }}</ref><ref>{{Cite journal |last1=Davis |first1=Donald R. |last2=Chapman |first2=Clark R. |last3=Durda |first3=Daniel D. |last4=Farinella |first4=Paolo |last5=Marzari |first5=Francesco |date=1996 |title=The Formation and Collisional/Dynamical Evolution of the Ida/Dactyl System as Part of the Koronis Family |url=https://linkinghub.elsevier.com/retrieve/pii/S0019103596900470 |journal=Icarus |language=en |volume=120 |issue=1 |pages=220–230 |doi=10.1006/icar.1996.0047|bibcode=1996Icar..120..220D |url-access=subscription }}</ref>

Dactyl was the first [[asteroid moon]] discovered. Its discovery settled the long debate over the existence of asteroid moons.

==See also==
*[[List of geological features on 243 Ida and Dactyl]]

==References==
{{Reflist}}

===Works cited===
{{refbegin|2}}
* {{cite journal |last1=Byrnes |first1=Dennis V. |author2=D'Amario, Louis A. |author3=Galileo Navigation Team |title=Solving for Dactyl's Orbit and Ida's Density |journal=The Galileo Messenger |issue=35 |date=December 1994 |url=http://www2.jpl.nasa.gov/galileo/mess35/DACTYL.html |archive-url=https://web.archive.org/web/19970105233902/http://www.jpl.nasa.gov/galileo/mess35/DACTYL.html |url-status=dead |archive-date=1997-01-05 |access-date=2008-10-23 |ref=CITEREFByrnesD'Amario1994}}
* {{cite journal |last=Chapman |first=Clark R. |author2=Klaasen, K. |author3=Belton, Michael J. S. |author4= Veverka, Joseph |title=Asteroid 243 IDA and its satellite |journal=Meteoritics |volume=29 |page=455 |date=July 1994 |bibcode=1994Metic..29..455C |ref=CITEREFChapmanKlaasenBeltonVeverka1994}}
* {{cite journal |last=Petit |first=Jean-Marc |author2=Durda, Daniel D. |author3=Greenberg, Richard |author4=Hurford, Terry A. |author5=Geissler, Paul E. |title=The Long-Term Dynamics of Dactyl's Orbit |journal=Icarus |volume=130 |issue=1 |pages=177–197 |date=November 1997 |doi=10.1006/icar.1997.5788 |bibcode=1997Icar..130..177P |ref=CITEREFPetitDurdaGreenbergHurford1997 |citeseerx=10.1.1.693.8814}}
{{refend}}

==External links==
*[http://www.nineplanets.org/ida.html 243 Ida and Dactyl] from ''The Nine Planets''
*[https://www.solarviews.com/eng/ida.htm Asteroid Ida and Dactyl] from ''Views of the Solar System''
*[http://www.cfa.harvard.edu/iauc/05900/05948.html IAUC 5948] announcing Dactyl's discovery
*[http://www.cfa.harvard.edu/iauc/06000/06082.html#Item2 IAUC 6082] announcing Dactyl's naming
*[https://near.jhuapl.edu/Voyage/img/Ida_dactyl.gif Ida and Dactyl] in false coloring, to reveal surface differences

{{Solar System moons (compact)}}

[[Category:Asteroid satellites]]
[[Category:Minor planets visited by spacecraft]]